1. Field of the Invention
The present invention relates to a piezoelectric resonator and more particularly to technology being able to be effectively used for improvement of a Q (Quality Factor) characteristic (that is, impedance ratio=anti-resonant resistance value/resonant resistance value), in particular.
2. Description of the Related Art
In a piezoelectric resonator such as a crystal resonator or a like using a bulk acoustic wave (BAW), a thickness of a film of its upper electrode is related to improvement of a Q characteristic to acquire a good frequency characteristic. When a wavelength of a standing wave occurring in the vicinity of a resonant frequency in a frequency band being used is defined as “λ”, an upper electrode having a film thickness of about 0.06λ to 0.15λ is used.
To make large a film thickness of the upper electrode is effective in controlling an effective coupling coefficient or a like. However, since the Q characteristic is degraded as the film thickness of the upper electrode becomes large, in general cases, the film thickness of the upper electrode to be used is not far out of the range described above.
Moreover, improvements of a resonant characteristic in a piezoelectric resonator are described in U.S. Pat. No. 6,472,954.
However, if a film thickness of an upper electrode in a piezoelectric resonator is made small, electric resistance of a thin wiring layer manufactured by the same processes as those used for the upper electrode is also increased.
A thickness of the wiring layer can be made large in processes which are added after fabrication using a process being different from those used for the upper electrode or using the same process as used for the upper electrode, however, entire processes are increased, unfavorably causing an increase in manufacturing costs.
These problems occur commonly not only in a resonator but also in a filter or a like using the resonator.